1. Field of the Invention
The present invention relates to a thread rubber composition and a golf ball using the thread rubber composition.
2. Description of the Background Art
Golf balls are generally classified into two categories: thread-wound golf balls having a center wound with thread rubber as its core material which is enveloped by a cover; and solid golf balls having a core material of solid rubber which is enveloped by a cover. The thread-wound golf balls are superior in shot feeling and controllability, while the solid golf balls are superior in flight performance and durability.
The thread-wound golf balls exhibiting such favorable controllability have often been employed by advanced amateur golfers and professional golfers. However, ordinary amateur golfers are reluctant to use them because of their inferior flight performance. Thus, improvement of the flight performance (impact resilience) of the thread-wound golf balls has been a critical issue.
U.S. Pat. No. 4,353,557 discloses a conventional technique to increase the impact resilience of the thread-wound golf balls by forming a thread rubber with a rubber composition having 2-20 parts by weight of carbon black blended with respect to 100 parts by weight of natural rubber and/or cis-1,4-polyisoprene.
Japanese Patent Publication No. 5-41272 discloses another conventional technique to employ a thread rubber including isoprene-butadiene random copolymer having a cis-1,4 bond content of at least 80 wt % as a rubber component.
With these techniques, however, the thread rubber would break during a winding step, and therefore, sufficient hardness for a golf ball would not be obtained.
When the thread rubber is wound around the center during the manufacturing process of a golf ball, it is generally elongated by 800 to 1100%. If the thread rubber breaks at this time, productivity will be degraded considerably. Thus, prescribed elongation percentage and strength are required for the thread rubber. Further, when the thread rubber is elongated, a prescribed tensile resilience is required so as to keep sufficient hardness for the golf ball. When the thread rubber is elongated, rubber molecules come to be orientated uniformly in the elongated direction, so that crystallization takes place. As a result, rubber elasticity is lost, thereby decreasing the impact resilience of the golf ball. Such decrease of the impact resilience should be suppressed.
The present invention is directed to solve the above-described problems. An object of the present invention is to provide a thread rubber exhibiting good impact resilience and reduced in breakage during a winding step, and a golf ball employing such a thread rubber.
According to an aspect of the present invention, a thread rubber composition for a golf ball has 0.5-0.7 parts by weight of sulfur and 0.5-5.0 parts by weight of vulcanization accelerator blended with respect to 100 parts by weight of a rubber component that is formed of either one of polyisoprene rubber, natural rubber and polybutadiene rubber, or a mixture thereof. The rubber component has a cis-1,4 bond content of from 90.0% to 93.8%, and the blended amount of sulfur is made greater than the blended amount of vulcanization accelerator. The rubber component preferably has a trans-1,4 bond content in a range between 2.0% and 5.0%.
According to another aspect of the present invention, a thread rubber is obtained by shaping the above-described thread rubber composition into a sheet through an extrusion step and a calendering step, vulcanizing the sheet to obtain a vulcanized sheet, and cutting the vulcanized sheet into threads.
According to a further aspect of the present invention, a golf ball is obtained by winding a thread rubber around a center and enveloping the center with a cover. The thread rubber is obtained by shaping the above-described thread rubber composition into a sheet, vulcanizing the sheet, and cutting the vulcanized sheet into threads.
Preferably, the sheet of the thread rubber composition has a ratio (t/T) of a thickness (t) of thin sheet material after the calendering step with respect to a thickness (T) of sheet after the extrusion step of from 0.03 to 0.5. Still preferably, the thickness of the thin sheet material after the calendering step is 0.5xc2x10.1 mm with a standard deviation "sgr" of not greater than 0.02.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.